1. Field of the Invention
This invention relates, in general, to non-integer programmable counters and more particularly to an I.sup.2 L programmable counter having a high frequency precision that uses feedback to adjust count length.
2. Background Art
Dual tone multifrequency (DTMF) dialers in telephone systems typically include a frequency generator or clock, a "divide-by-N" counter, a "divide-by-16" counter, a decoder, and a digital-to-analog (D/A) converter. The clock typically comprises a quartz crystal that produces a signal having a frequency of 3.5 megahertz. The "divide-by-N" counter divides the signal by some integer N to provide a signal having a frequency in the range desired. The "divide-by-16" counter provides the input to the D/A converter through a suitable decoder. The output of the D/A converter is a 16 stepped sine wave. One alternative approach is to use a 16 bit shift register in place of the "divide-by-16" counter, eliminating the need for a decoder, however, the result is the same.
These previously known DTMF dialers have certain disadvantages such as poor accuracy, relatively high current requirements, and high costs. These disadvantages make the previously known circuits less than desirable, for example, when the circuit is incorporated in an electronic telephone chip using I.sup.2 L technology in order to operate at 1.1 volts.
Therefore, what is needed is a non-integer programmable counter where count length is varied to give a higher frequency precision, allowing for a less expensive input signal clock and having a reduced current requirement.